Graphite design, fabrication, test and analytical studies have shown that the use of high-stiffness, high-strength composites such as graphite/resin composites can reduce the weight of structural components by as much as 50%, improve structure efficiency, and provide significant benefits in cost and performance.
Accordingly, considerable interest has arisen in the use of graphite composites in both internal and external aircraft applications as well as in engine nacelles. For instance, in external applications such as in fairings, skins, rudders, stabilizers, flaps and doors, as well as in engine nacelles, it would be desirable for the resin to resist burning and also prevent short lengths of the graphite fiber from spreading to other areas. For internal applications such as cargo compartment liners and passenger compartment floor panels, it would be desirable to utilize a resin that eliminates or mitigates the level of flammability, smoke, and toxicity.
Further, in the nacelle, fire spreading from the hot zone through an acoustic panel fire wall could affect fuel lines, electrical equipment and hydraulic fluids, among other items. For this reason, the FAA has established a fire-safe requirement that any fire wall in an aircraft should be capable of withstanding a 2000° F. flame for 15 minutes.
Any organic material will burn if its ignition temperature is reached. However, extensive research has been conducted for a number of years to reduce the hazards from fire. In most cases, fire-retardant additives have been chosen. In other instances, polymer structures have been modified in order to reduce burning tendencies. Thus, the flammability of polymeric systems can be controlled by introducing elements which interfere with gas-phase oxidation reactions in the flame zone or modify the pyrolysis and decomposition reaction in the solid phase. Both bromine and chlorine are used extensively in the formulation of flame-retardant chemicals for polymers and operate by interfering with vapor phase combustion reactions. Organophosphorous compounds interfere with the combustion reaction in the condensed phase.
There are two general approaches to imparting flame retardance to materials, reactive and additive. Reactive fire-retardant chemicals enter into chemical reaction and become an integral part of the polymer structure; whereas the additive chemical is physically dispersed in the polymer but does not become part of the polymer structure. Generally, those compounds containing a halogen (usually chlorine or bromine), phosphorous, or nitrogen have been found to be effective flame retardants. Reactive flame-retardant intermediates are normally used in the manufacture of unsaturated polyesters, alkyds, epoxies and polyurethanes. However, the term “flame retardant” becomes moot. For instance, materials that are flame retardants at relatively low heat flux (e.g., a burning match) can have little or no effect in the high heat flux that accompanies a fuel spill or some other fire generating a 2000° F. flame.
Although a large class of brominated fire retardants has been used quite extensively, the toxic byproducts generated have resulted in their use being phased out. Hence, as a replacement of the brominated compounds, the phosphorylated compounds have been used increasingly. One class of phosphorylated compounds has been polyphosphinohydrazides generalized by the structure represented by Formula 1:
where R is H, alkyl, aryl, heterocyclic, or cycloaliphatic.
Although polyphosphinohydrazides according to Formula 1 have performed well and have been able to replace the brominated fire retardants, they have not done well above 2000° F. and are also greatly hydrophilic in nature. As such, polyphosphinohydrazides are easily washed out of composites and laminates using such materials upon contact with water. Accordingly there remains a need for a fire retardant that exhibits a similar or superior fire retarding capability while at the same time being resistant to dissolution by water. There also remains a need for a fire retardant that does not generate toxic products or leach out of the resin systems. Furthermore, there remains a need for a fire retardant that exhibits a fire retarding capability at or above 2000° F. that leaves a structurally stable char.